If you could clarify what you would expect them to do 'together', then I'd be somewhat grateful… from my perspective, Macs and iPads are mostly separate devices for separate use cases.
When getting a Mac mini, you should consider that you are going to require some kind of external display which you might need to purchase.
Generally, I would recommend any macOS-device for
- tasks that require bypassing the boundaries of iPadOS (a lot of desktop applications, starting with 'real' Microsoft Word, utilities like terminal, disk utility, …)
- the improved performance you can currently still can only get from a desktop computer (especially Mac mini)
- if you plan to use some Apple software, you can (as far as I know) use Final Cut Pro exclusively subscription-based, on iPadOS
When using both devices for the same task… well, you'll be syncing via cloud storage probably… still, I personally do not feel like starting to edit a photo in Lightroom on Mac and finishing on iPad. It's just not it; also, I think, it does not even work really. Same with DaVinci Resolve, which has absolutely great functionality on iPad, yet is still not comparable to the proper desktop version.
TL;DR from quick consideration, my perspective would be, the are not really supplementary, and more two devices.
I'd recommend a Mac for the performance and the software choices you get.
But maybe you can get another take.